doExactConfig.nix

  # This is a tiny bit better than doJailbreak.
  #
  # We essentially *know* the dependencies, and with the
  # full cabal file representation, we also know all the
  # flags.  As such we can sidestep the solver.
  #
  # Pros:
  #  - no need for doJailbreak
  #    - no need for jailbreak-cabal to be built with
  #      Cabal2 if the cabal file requires it.
  #  - no reliance on --allow-newer, which only made
  #    a very short lived appearance in Cabal.
  #    (Cabal-2.0.0.2 -- Cabal-2.2.0.0)
  #
  # Cons:
  #  - automatic flag resolution won't happen and will
  #    have to be hard coded.
  #
  # Ideally we'd just inspect the haskell*Depends fields
  # we feed the builder. However because we null out the
  # lirbaries ghc ships (e.g. base, ghc, ...) this would
  # result in an incomplete --dependency=<name>=<name>-<version>
  # set and not lead to the desired outcome.
  #
  # If we could still have base, etc. not nulled, but
  # produce some virtual derivation, that might allow us
  # to just use the haskell*Depends fields to extract the
  # name and version for each dependency.
  #
  # Ref: https://github.com/haskell/cabal/issues/3163#issuecomment-185833150
  # ---
  # ghc-pkg should be ${ghcCommand}-pkg; and --package-db
  # should better be --${packageDbFlag}; but we don't have
  # those variables in scope.
  doExactConfig = pkg: pkgs.haskell.lib.overrideCabal pkg (drv: {
    # TODO: need to run `ghc-pkg field <pkg> id` over all `--dependency`
    #       values.  Should we encode the `id` in the nix-pkg as well?
    preConfigure = pkgs.lib.traceSeq deps (drv.preConfigure or "") + ''
    configureFlags+=" --exact-configuration"
    globalPackages=$(ghc-pkg list --global --simple-output)
    localPackages=$(ghc-pkg --package-db="$packageConfDir" list --simple-output)
    for pkg in $globalPackages; do
      if [ "''${pkg%-*}" != "rts" ]; then
        configureFlags+=" --dependency="''${pkg%-*}=$pkg
      fi
    done
    for pkg in $localPackages; do
      configureFlags+=" --dependency="''${pkg%-*}=$pkg
    done
    #echo "<<< <<< <<<"
    #echo ''${configureFlags}
    configureFlags=$(for flag in ''${configureFlags};do case "X''${flag}" in
          X--dependency=*) echo ''${flag%=*}=$(ghc-pkg --package-db="$packageConfDir" --global field ''${flag##*=} id | awk -F': ' '{ print $2 }');;
          *) echo ''${flag};;
          esac; done)
    #echo "--- --- ---"
    #echo ''${configureFlags}
    #echo ">>> >>> >>>"
'';
  });